Mixer circuit

ABSTRACT

A mixer circuit includes a local frequency multiplication unit including a pair of transistors having bases receiving local oscillation waves inverted in phase. A reference transistor is differentially connected with the pair of transistors. The pair of transistors and the reference transistor have their emitters connected to a collector of a modulated wave input transistor having a base receiving a modulated wave signal and an emitter connected to a constant current source, and have their collectors connected to a load. The commonly connected collectors of the pair of transistors and the collector of the reference transistor output modulation signals inverted in phase. The sum of currents flowing through the pair of transistors and the reference transistor equals the constant current of the constant current source flowing through the modulated wave input transistor, and the mixer circuit has a gain.

TECHNICAL FIELD

The present invention relates generally to mixer circuits andparticularly to a direct conversion mixer for use in mobilecommunications.

BACKGROUND ART

A direct conversion system is known which converts a signal modulatedinto a radio frequency directly to a baseband signal and modulates abaseband directly to a radio frequency. Generally, this system does notprovide an intermediate frequency. As such, it can dispense with imagesuppression, reduce spurious source, an d simplify a systemadvantageously. It is also known, however, that the system provides 2ndorder intermodulation distortion, local oscillation wave noise, andother similar disadvantages. One approach to alleviate thesedisadvantages is an even harmonics mixer such as an anti-parallel diode.

In recent years there is a demand for miniature and inexpensive ICs forhigh-frequency radio communications, as represented for example bymobile phones. In such a market, current consumption reduction, ICyields and the like are important issues.

In an even harmonics mixer, a mixer circuit itself does not consume acurrent. To operate the mixer circuit, however, a high local oscillationwave input level is required. Accordingly, overall current consumptionreduction has been an issue to be addressed. Furthermore there is also ademand for reducing an influence of semiconductor process variation.

As an approach to overcome such disadvantages, a system using atransistor's diode characteristics has been considered in recent years.

FIG. 5 is a circuit diagram showing one example of a conventional evenharmonics mixer using transistors. As shown in the figure, transistors 1and 2 have their respective collectors connected together and theirrespective emitters connected together to configure a local frequencymultiplication unit. A resistor R1 is connected between the collectorsand a power supply and a resistor R2 is connected between the emittersand ground. Transistors 1 and 2 receive a modulated wave signal BB attheir respective emitters and receive signals LO and ↓LO (“↓” indicatesan inverted signal) each having a component of a frequency of a localoscillation wave inverted in phase at their respective bases. Thus thelocal frequency multiplication unit generates a local oscillation wavefrequency component multiplied by two and can extract a desiredfrequency component by multiplying an input signal frequency and thelocal oscillation wave frequency component.

The mixer of FIG. 5, however, is associated with a small power gain andrequires a high local oscillation wave input to operate the localfrequency multiplication unit.

DISCLOSURE OF THE INVENTION

A main object of the present invention is therefore to provide in thefield of high-frequency radio communications a mixer circuit suitablefor applications for which there is a demand for low frequency currents,high IC yields and simplified system configurations.

In accordance with the present invention a mixer circuit includes: alocal frequency multiplication unit configured of a first transistorhaving an input electrode receiving a frequency component of a localoscillation wave signal and a second transistor receiving a signalopposite in phase to the local oscillation wave, the first and secondtransistors having their respective first electrodes connected togetherand their respective second electrodes connected together, the localfrequency multiplication unit outputting a modulation signal at thefirst electrode of each transistor; a third transistor connected in apair with each of the first and second transistors of the localfrequency multiplication unit, having an input electrode receiving areference signal, and a first electrode differentially outputting themodulation signal; a voltage source providing the first, second andthird transistors at their respective first electrodes with apredetermined voltage; a signal input unit connected to the first,second and third transistors at their respective second electrodes andreceiving a modulated signal; and a constant current source connected tothe signal input unit.

Furthermore, the signal input unit is a fourth transistor having aninput electrode receiving the modulated signal, a first electrodeconnected to the second electrode of each of the first, second and thirdtransistors, and a second electrode connected to the constant currentsource.

Furthermore, there are provided two sets of the local frequencymultiplication unit, the third transistor for reference and the fourthtransistor, the voltage source is connected to the two local frequencymultiplication units commonly, and the constant current source suppliesa constant current to the two fourth transistors commonly.

Furthermore, the constant current source is a variable constant currentsource.

Furthermore, the mixer circuit further includes a variable currentsource varying a bias current provided to the input electrode of thethird transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a mixer circuit of one embodiment of thepresent invention;

FIG. 2 is a circuit diagram of another embodiment of the presentinvention;

FIG. 3 is circuit diagram showing another embodiment of the presentinvention;

FIG. 4 is a circuit diagram showing still another embodiment of thepresent invention; and

FIG. 5 is a circuit diagram showing one example of an even harmonicsmixer using a conventional transistor.

BEST MODES FOR CARRYING OUT THE INVENTION

FIG. 1 is a circuit diagram of a mixer circuit of one embodiment of thepresent invention. In FIG. 1, similarly as shown in the conventionalexample of FIG. 5, transistors 1 and 2 have their respective collectorsconnected together and their respective emitters connected together,each collector receiving a power supply via a resister R3, and theirrespective bases receiving frequency components signals LO and ↓LO of alocal oscillation wave to configure a local frequency multiplicationunit 10. Local frequency multiplication unit 10 generates in a mannersimilar to that of an even harmonics mixer a frequency corresponding tothe local oscillation wave frequency multiplied by two.

Furthermore, a reference transistor 3 has its emitter connected to theemitters of transistors 1 and 2, its collector connected to a powersupply via a resistor R4, and its base receiving a bias current Ref.

Resistors R3 and R4 configure a load 6 serving as a voltage source. Inplace of resistors R3, R4 a different type of load may be connected.Transistors 1, 2 and 3 have their emitters connected to the collector ofa transistor 4 for receiving a modulated wave. Transistor 4 has itsemitter receiving a constant current from a constant current source 5and its base receiving a modulated wave signal BB.

In the mixer circuit of FIG. 1, the sum of the current flowing throughlocal frequency multiplication unit 10 configured of transistors 1 and 2and that flowing through transistor 3 equals that flowing throughtransistor 4. Furthermore, the current flowing through transistor 4equals the constant current of constant current source 5. As such byallowing transistor 4 to have a gain a mixer circuit having a gain canbe implemented.

The frequency multiplied by two and generated by local frequencymultiplication unit 10 configured of transistors 1 and 2 varies thecurrent of local frequency multiplication unit 10. Since transistor 4provides a constant-current operation relative to local frequencymultiplication unit 10 and transistor 3, the variation in currentresults in transistor 3 having a current of opposite phase flowingtherethrough. Transistor 4 has a gain for a modulated wave that isreceived and the amplified signal is converted in frequency by localfrequency multiplication unit 10. The signal converted in frequency isoutput at load 6 as differential output signals RF and ↓RF.

Thus in the present embodiment a mixer circuit configured of localfrequency multiplication unit 10 configured of transistors 1 and 2 andan input unit formed of transistor 4 to receive a modulated wave isprovided with reference transistor 3 paired with local frequencymultiplication unit 10 to provide a differential mixer. Withoutincreasing an input level of a local oscillation wave a large-gain mixercircuit can be implemented.

FIG. 2 is a circuit diagram of another embodiment of present invention.In FIG. 2, local frequency multiplication unit 10 and transistors 3 and4 are similar in configuration to those shown in FIG. 1. A localfrequency multiplication unit 20 and transistors 9, 11 are configuredsimilarly as shown in the mixer circuit of FIG. 1. More specifically,transistors 7 and 8 have their respective collectors connected together,their respective emitters connected together, and their respective basesreceiving frequency component signals LO, ↓LO of a local oscillationwave, respectively.

Transistors 7, 8 have their respective emitters connected to the emitterof a reference transistor 9. Reference transistor 9 has its basereceiving a bias current. Each transistor 7, 8, 9 has its emitterconnected to the collector of a transistor 11 receiving a modulatedwave. Transistor 11 has its base receiving a modulated wave signal ↓BBopposite in phase to the modulated wave signal BB input to the base oftransistor 4.

Local frequency multiplication unit 20 has its collectors connected tocollectors of local frequency multiplication unit 10, and each collectorreceives a power supply commonly via a resistor R3. Transistor 9 has itscollector connected to a collector of transistor 3 and these collectorsreceive a power supply via a resistor R4. Transistor 11 has its emitterconnected to an emitter of transistor 4 and these emitters are connectedto constant current source 5 commonly. In other words, the mixer circuitof FIG. 2 corresponds to the mixer circuit of FIG. 1 if the mixercircuit of FIG. 1 has the Gilbert cell configuration.

Thus in the present embodiment each mixer circuit having the sameconfiguration as shown in FIG. 1 can have a large gain, asaforementioned, and it can also realize the Gilbert cell configurationto implement a mixer circuit capable of suppressing a frequencycomponent of a local oscillation wave leaking toward an output.

FIG. 3 is a circuit diagram showing another embodiment of the presentinvention. In the present embodiment, constant current source 5 shown inFIG. 1 is replaced by a variable current source 12. A variable gaincontrol effect can be obtained.

FIG. 4 is a circuit diagram showing still another embodiment of thepresent invention. In the present embodiment, a variable current source13 is connected to the base of the reference transistor 3 of FIG. 1 toallow a bias current to be variable. A variable gain control effect canbe obtained.

The above embodiments have described a mixer circuit mixing a frequencycomponent of a local oscillation wave and the signal of a modulated waveBB together to output a signal of a modulated wave. The mixer circuitmay receive a radio frequency signal having been received as a signal ofa modulated wave and mix it with a frequency component of a localoscillation wave, and output a baseband signal as a modulation signal.

Furthermore while the above embodiments are configured of a bipolartransistor, they may be configured of a field effect transistor.

Thus in the present invention a mixer circuit configured of a localfrequency multiplication unit and an input unit receiving a modulatedwave is provided with a reference transistor to provide a differentialmixer thereby to simplify a system configuration with large gain.Furthermore, constant-current control can reduce dependency onsemiconductor process variation and providing a constant current sourcein a bandgap configuration can reduce power supply voltage variation andtemperature variation.

INDUSTRIAL APPLICABILITY

In accordance with the present invention a mixer circuit can have bothof an advantage of a conventional mixer circuit and that of an evenharmonics mixer to allow the even harmonics mixer to be active toimplement a large-gain mixer circuit while preventing an influence of acarrier component. It is applicable to mobile communications involvingmodulation from a baseband signal directly to a radio frequency.

What is claimed is:
 1. A mixer circuit comprising: a local frequencymultiplication unit configured of a first transistor having an inputelectrode receiving a frequency component of a local oscillation wavesignal and a second transistor receiving a signal opposite in phase tosaid local oscillation wave, said first and second transistors havingtheir respective first electrodes connected together and theirrespective second electrodes connected together, said local frequencymultiplication unit outputting a modulation signal at said firstelectrode of each said transistor; a third transistor connected in apair with each of said first and second transistors of said localfrequency multiplication unit, having an input electrode receiving areference signal, and a first electrode differentially outputting saidmodulation signal; a voltage source providing said first, second andthird transistors at their respective first electrodes with apredetermined voltage; a signal input unit connected to said first,second and third transistors at their respective second electrodes andreceiving a modulated signal; and a constant current source connected tosaid signal input unit.
 2. The mixer circuit according to claim 1,wherein said signal input unit is a fourth transistor having an inputelectrode receiving said modulated signal, a first electrode connectedto said second electrode of each of said first, second and thirdtransistors, and a second electrode connected to said constant currentsource.
 3. The mixer circuit according to claim 1, wherein: there areprovided two sets of said local frequency multiplication unit, saidthird transistor for reference and said fourth transistor; said voltagesource is connected to said two local frequency multiplication unitscommonly; and said constant current source supplies a constant currentto said two fourth transistors commonly.
 4. The mixer circuit accordingto claim 2, wherein said constant current source is a variable constantcurrent source.
 5. The mixer circuit according to claim 1, furthercomprising a variable current source varying a bias current provided tosaid input electrode of said third transistor.